Abstract [en]

We used nitrogen stable isotopes to study the regulation of nitrogen fixation by filamentous cyanobacteria. Nitrogen fixation was found to be almost insensitive to combined nitrogen, along a gradient from the Himmerfjarden sewage treatment plant discharge to the open sea. We found similarly low cyanobacterial (mostly Aphanizomenon sp.) delta N-15-values at all stations, despite significant differences along the bay in both total nitrogen concentrations in water and delta N-15 in seston (particles <10 mu m), the latter used as a proxy for algae growing on combined nitrogen alone. Only late in the productive season, when cyanobacterial biomass was declining or already low, did elevated delta N-15 suggest uptake of combined nitrogen. However, this coincided with an increase in the contribution of Dolichospermum spp. to overall diazotrophic biomass and may indicate uptake of combined nitrogen by this species. These results indicate that almost all nitrogen used for growth by nitrogen-fixing cyanobacteria in the study area comes from nitrogen fixation, and very little from uptake of dissolved combined nitrogen. This study was part of a whole ecosystem experiment analyzing the effects of nitrogen removal in a sewage treatment plant discharging to the Himmerfjarden Bay, northern Baltic Sea Proper.

Zakrisson, Anna

Abstract [en]

The overall aim of the study was to provide better insights to the ecological role and impact of cyanobacteria in Baltic Sea (BS) bay, coastal and open sea areas. Biomass and heterocyst development of diazotrophic, heterocystous cyanobacteria were monitored over several years simultaneously as physical parameters such as nutrients and temperature. Nitrogen fixation was estimated as well as its transfer in the BS food web. Even after decades of debate there is still controversy whether eutrophication of lakes and estuaries/coastal areas should be managed by reducing phosphorus only or also nitrogen. Central to this debate is whether nitrogen fixation by cyanobacteria can replace shortages of combined nitrogen quickly enough to make phosphorus the limiting nutrient and nitrogen removal pointless or even harmful. Also, it is not clear if available combined nitrogen inhibits heterocystous cyanobacterial nitrogen fixation and if it is used for their growth in situ. A large ecosystem-wide experiment started in Himmerfjärden bay in year 1997, where the N-loadings and release depth from a modern sewage treatment plant (STP), located in the inner part of the bay, were modulated. The STP creates a steep gradient of nutrients and stable nitrogen isotopes, which can be used to study uptake of combined nitrogen, as well as biomass development and primary productivity. A 35-year long data series was used to achieve good insights into phytoplankton development and primary productivity in the Baltic Sea over the last couple of decades. These in vivo long time series, based on monitoring data, in combination with shorter series (2-3 seasons, including measurements of colony stoichiometry and stable isotopes), have resulted in a unique meta-dataset, allowing for high-resolution observations into the role of the cyanobacteria in the Baltic Sea ecosystem.